In the proteomics era, minimizing the time required to perform the necessary NMR assignment and structure studies is key to progress. Such studies often require months of NMR time for protein molecular weights in excess of 30 kDa. Moreover, strategies aimed at streamlining studies of large proteins and macromolecular complexes include the use of higher magnetic fields and partial deuteration. Consequently, spin-lattice relaxation times (T1) become inordinately long and sensitivity (or time) is compromised. Paramagnetic relaxation agents may be used to quench solvent T1's, without undue line broadening of protein resonances. If T1 of water is sufficiently low, a considerable reduction of 1H relaxation times is expected to occur on the surface and even the interior of the protein through water exchange and spin-diffusion. In flow-through NMR one of the most serious limitations to sensitivity is the polarization time (ie the sample will become polarized over a timescale comparable to 3×T1, which is often far less than the time a sample spends in the magnet). As a result, NMR is frequently the bottleneck in tandem imaging techniques such as tandem HPLC/Mass Spectroscopy/flow NMR methods. Paramagnetic relaxation agents are equally important in Magnetic Resonance Imaging (MRI), since relatively small quantities can be safely introduced intravenously or orally, resulting in significant spin-lattice relaxation or T1 effects in certain tissues. Differences in T1 effects in neighboring tissues give contrast in MRI images, particularly when pulse sequences are employed in which differences in T1 are exploited. A second means of enhancing contrast by MRI is by so-called T2-imaging. This requires relaxation or contrast agents which contribute to local changes in susceptibility or T2 (spin-spin relaxation times).
Paramagnetic Relaxation Agents and MRI Contrast Agents
Conventional paramagnetic relaxation agents are generally not used in NMR applications due to: 1) weak association of certain regions of the protein with the agent (for example a Gd3+ chelate), 2) severe line broadening, and 3) difficulty of later separating the sample from the paramagnetic additive.
In theory, Gd3+ is a good paramagnetic relaxation agent because of its large magnetic moment and ns-timescale electronic spin relaxation rates. Simple Gd3+-chelates such as EDTA, DTPA, or DOTA are routinely used as paramagnetic relaxation agents in NMR studies. However, there are numerous shortcomings. Firstly, the chelated gadolinium ion may coordinate with the carboxyl ligands of the chelate, with water, and to some extent, with regions of partial charge on the protein, thereby excessively shifting and broadening some protein resonances and obscuring assignments. Secondly, rapid tumbling of a small chelate such as Gd3+:EDTA, Gd3+:DTPA, or Gd3+:DOTA diminishes the effectiveness of the relaxation agent, since the correlation time, τeff, associated with paramagnetic relaxation, is a function of the average tumbling time of the Gd3+ chelate, τc, and the electronic relaxation time of the paramagnet, T1e, such that1/τeff=1/τc+1/T1e.
In order to overcome this deficiency, a wide variety of slow-tumbling water soluble complexes that bind several to many Gd3+ ions have been designed, primarily for purposes of obtaining enhanced bulk water relaxation in Magnetic Resonance Imaging (MRI) applications. In this case, the Gd3+ complexes dramatically decrease bulk relaxation times and so-called T1-weighted images may reveal improved contrast, depending on the partitioning properties of the contrast agent into adjacent tissues or cells. Such complexes include zeolites,i micellar aggregates,ii polyaminoacids,iii polysaccharides,iv and dendrimers.v,vi While these complexes are useful for specific T1-weighted MRI applications in certain tissues, many of these materials would be problematic as relaxation agents in high throughput NMR or NMR studies of proteins due to nonspecific binding between chelate and the molecule of interest and background signal from the agent. Furthermore, although the majority of NMR experiments in assignment studies may benefit from a relaxation agent, others such as long-range NOESY distance measurements, or relayed experiments which require long lived coherences, may perform worse with the addition of relaxation agents. Consequently, in NMR experiments, it is highly desirable that the relaxation agent can be reliably and rapidly removed and the molecule under study easily retrieved. In MRI imaging experiments it is also important that the contrast agent also be removed. Nanoparticles of 50 nm diameters or smaller, are routinely removed via the reticuloendothelial system and ultimately excreted via the bile.vii, viii, ix 
Basic Physical Features of the GdF3 and LaF3/GdF3 Nanoparticles
Nanoparticles have found many uses in wide ranging fields of materials chemistry, including catalysts, energy storage, packaging and textiles, and advanced computing. In the health science industry, nanoparticles are used both as detection and delivery agents. For example, hydrophobic antitumor drugs can be packaged within nanoparticle matrices that commonly take the form of inorganic or organic polymers, proteins or antibodies, dendrimers, or liposomes. In some cases, the nanoparticle may be functionalized for targeting to a specific tissue or cell, by additional ligation. For example, coatings such as lipopolysaccharides are frequently added to control drug release, endosomal uptake, and bioactivity.
There is a need for contrast and relaxation agents that have the following attributes.
1) Strong relaxation effects without undue broadening. The electronic relaxation time should be on the nanosecond timescale and the paramagnetic complex should be large enough to inhibit excessively fast tumbling. In MRI, strong (T1) relaxation effects are also desired.
2) High solubility. This helps to extend the upper limits of concentration and thus contrast in MRI, while relaxation effects in NMR also depend on concentration. The particles can also be readily prepared to be soluble in a wide variety of solvents.
3) Can be readily functionalized and with a multitude of groups. Dendrimers, zeolites, and polypeptides, for example, can be problematic if multiple ligands are needed. In MRI multiple groups could be envisaged to assist in directing a contrast agent to a target tissue, providing proper charge or solubility, controlling immune responses by appropriate coatings such as PEG, and ligating attached drugs for subsequent release.
4) Do not exhibit leaching effects. Traditional lanthanide chelates will leach (leak) Gd3+ ions over time. Since Gd3+ is toxic this is an obvious limitation for MRI. In NMR, this is a limitation because free lanthanides may then bind to the protein or molecule of interest.
5) Can be easily retrieved or removed. Centrifugation or ultracentrifugation are often used to separate or fractionate particles of different densities. Dialysis can be used to size fractionate particles. Other techniques such as binding agents, including, for example, but not limited to antibodies, and biotin can be used to extract particles by first coating the particle with antigen and avidin, respectively.
It is an object of the present technology to overcome the deficiencies in the prior art.